Low temperature properties such as superconductivity and superfluidity are now widely used in a range of different applications including Magnetic Resonance Imaging (MRI), superconducting magnets, sensors and in fundamental research. Historically, the evaporation of cryogenic liquids such as nitrogen or helium has been used as a cooling mechanism in order to reach the low temperatures required for such applications. Cryogenic liquids have associated disadvantages in that they are often “consumable” due to leaks within associated apparatus such as “in situ” liquefiers or storage vessels. Furthermore such apparatus for storing or otherwise handling cryogenic liquids is often bulky and requires special handling procedures. Such apparatus and procedures are somewhat incompatible with patient care environments.
More recently, closed cycle refrigerators (CCR) have been used to replace cryogenic liquids in providing an alternative refrigeration mechanism. In contrast with the evaporation of cryogenic liquids, CCRs do not rely upon a phase change within the coolant. Indeed, CCRs operate upon a principle of using the cooling which is associated with the work of compression and expansion of a working gas coolant. Accordingly, the use of CCRs is of particular interest for cooling apparatus for medical applications, such as MRI. The term “mechanical refrigerators” is used herein to describe such apparatus although those of ordinary skill in the art will appreciate that the term “cryocooler” is synonymous with this term.
Mechanical refrigerators use a working gas such as helium to provide cooling at relatively modest cooling powers, to a temperature of 2 to 20 Kelvin. Mechanical refrigerators are extremely advantageous since they are closed systems with few moving parts and are essentially lossless with regard to the working gas. For these reasons, they are attractive both technologically and commercially and there is an on-going desire to improve the performance of such mechanical refrigerators.
Despite advances which have been made to date in the technology associated with mechanical refrigerators, the thermodynamic coefficient of performance (COP) and the associated cooling efficiency of such mechanical refrigerators are still rather unsatisfactory. As an example, an input electrical power of up to around 8 kiloWatts is needed in order to provide a cooling power of around 1 Watt at the liquid helium temperature of 4.2 Kelvin. There are numerous applications, such as the cooling of superconducting magnets (for use in MRI for example) or the cooling of relatively high thermal masses, where the cooling time required to cool from room temperature to the low temperature regime is an important parameter. It will be appreciated that it is desirable to reduce this cooling time to as short a period as possible. This is particularly desirable in MRI applications in patient care environments where long cooling down periods are unacceptable.
One technique developed by the present applicant to reduce the cooling time involves the introduction of a heat pipe between two cooled stages of the mechanical refrigerator. This technique is described fully within our United Kingdom patent application number 1119846.2, the contents of which are incorporated herein by reference.
The present applicant has continued developing techniques for improving upon the cooling time in such systems and it is in this context that the present invention finds application and provides new advantages.